When a battery is used for powering an analog circuit, a voltage switching converter may be used for DC-DC conversion for switching a voltage level provided by the battery to an appropriate level for powering the analog circuit. However, the voltage switching converter may introduce ripples in the output voltage, which may be harmful to sensitive analog circuits.
Low dropout (LDO) voltage regulators are commonly used as filters between a voltage switching converter and an analog circuit in order to remove ripples from the output voltage supplied to the analog circuit.
Typically, a LDO voltage regulator comprises an output device and a differential amplifier, which receives a fraction of the output voltage signal and a stable reference voltage. If the output voltage differs from the reference voltage, the power to the output device is changes to maintain a constant output voltage.
Preferably, LDOs should consume very little power. However, ultra-low-power LDOs have limitations in bandwidth, which result in stability issues, as well as bad transient performance and low power supply rejection ratio (PSRR).
An output stage may be arranged in between the error amplifier and the output device to improve performance of the LDO. Also, or alternatively, adaptive biasing may be used in order to make a LDO bias current proportional to a load current.
In EP1635239, circuits and methods to achieve dynamic biasing for the complete loop transfer function of a current mode voltage regulator are disclosed. The circuit comprises a Mirror-Transconductor Amplifier type operational transconductance amplifier (OTA) wherein its transconductance is linearly dependent on its biasing current. This biasing current is a linear derivative of the OTA's output current. A current amplification circuit couples the regulator output current linearly with said OTA's output current. In this configuration the iterative biasing of the OTA forms a feed-forward loop, which contains a low-pass filter for stability and a negative feedback loop is closed by connecting the regulator voltage output to the OTA input.
However, many relevant parameters of the error amplifier, such as DC-gain, settling error and PSRR are directly related to the biasing of the error amplifier. Adaptive biasing of the error amplifier will unavoidably worsen those parameters over the full range. Additionally, kickback on the reference voltage will be a severe issue in ultra-low-power applications, as the input reference will be a relatively high impedance node.